The present invention relates to flexible hoses having a reinforcement which includes continuous metal element such as wires, stranded wires or cables which extend from one flange to the other.
The invention is especially applicable to hoses having so-called integrated flanges. Integrated flanges are placed in position and connected to the reinforcement of the hose when the hose is being manufactured. When the hose has been vulcanised, the flanges become integralised therewith.
When the hoses have reinforcements containing continuous metal elements and are provided with metal flanges connected to such reinforcements, their electrical resistivity measured from flange to flange is extremely low. For example, certain hoses intended to convey petroleum products under difficult conditions have a reinforcement formed from metal cables and are provided with integrated flanges. The ohmic resistance of such hoses, measured from flange to flange, is extremely low, of the order of a few thousandths of an ohm. However, so that there shall be no dange of sparking and thus of explosion, or so that the cathodic protection against corrosion provided for the installations will not be interfered with, it appears desirable for hoses intended for applications of this kind to have an electrical resistance between their flanges which, when measured on an ohmmeter, is greater than 25,000 ohms, or again which is such that an electric lamp will not light when an electrical circuit is set up comprising the hose, the said lamp and a 4.5 volts source of DC electrical current.
It is an object of the invention to overcome or minimise these drawbacks and provide a flexible hose whose electrical resistance is sufficiently high for there to be no risk of sparking even if the reinforcement of the hose includes continuous metal elements which extend from one flange to the other and even if these metal elements come into contact with the flange or come very close to it.
The invention is based on the surprising discovery that if the parts of the metal flange adjacent the metal elements of the reinforcement are covered with a layer of a resin which is known to be an electrical insulator, the electrical resistance of the hose remains low, but on the other hand it becomes extremely high and reaches the required levels if a fibrous material of high mechanical strength and high electrical resistivity is associated with the resin.